80% of U.S. women use oral contraceptives (OC) for an average duration of 5 years. There is a large gap between lowest expected failure rates of about 1.5% and typical use failure rates of about 7% per year. This gap may be due to incorrect use or to decreased OC effectiveness in population subgroups. Recent reports suggest greater oral contraceptive failure among heavier women, particularly those using the lowest doses. The prevalence of obesity in the U.S. population has recently increased to about 23% in women aged 20-29, peak years for OC use. Obesity is most prevalent among poor and minority women who already experience higher rates of contraceptive failure. During recent decades, the typical OC dose has decreased by about 75%. OC physiology and effectiveness have not been evaluated in obese women.This is a revised proposal in which we propose a double blind randomized clinical trial to include about 100 normal weight women and 100 obese women with normal ovulatory function at baseline; we will randomize these participants to widely used monophasic oral contraceptives (30ugEE/150ug LN or 20ugEE/100ug LN). Participants will undergo evaluation to detect ovarian follicle development and ovulation during OC use; we will use transvaginal sonograms to assess ovaries and endometrium and will measure circulating endogenous hormones (progesterone). Participants will record bleeding and adverse events using a diary. We will use this data to assess whether obese women are at higher risk of oral contraceptive failure than normal weight women due to less ovarian suppression. We will be able to assess differences by weight and by dose and evaluate weight-dose subgroups. We will be able to assess risk of pregnancy based on development of pre-ovulatory follicles and ovulation. This study will have very good power to detect clinically important differences. Results could directly and immediately affect contraceptive practice.